SOME EXTINCT ARGENTINE MAMMALS 89 



of plates, in the glyptodons the whole structure is welded 

 into a single piece. It must not, however, be supposed that 

 this carapace consists of a single solid dome of bone, as, 

 if it did, there would, of course, be no possibility of growth. 

 On the contrary, the carapace is composed of polygonal 

 or rhomboidal plates articulating at their edges, and thus 

 allowing of free growth. In very old individuals a consider- 

 able number of these plates may, however, become com- 

 pletely fused together. During life these bony plates were 

 covered with small horny shields, as in the living arma- 

 dillos, and they frequently show incised lines formed by the 

 lines of union between such shields. For instance, in the 

 members of the typical genus of the group, or ring-tailed 

 glyptodons, each bony plate was smooth and polygonal in 

 shape, while the lines indicating the borders of the horny 

 shields take the form of a rosette. Another important 

 point of difference from the armadillos is to be found in 

 the contour of the skull, which is short, deep, and rounded, 

 instead of being long, flattened, and pointed at the muzzle. 

 Then, again, whereas the armadillos have small cylindrical 

 teeth, those of the glyptodons are large and fluted at the 

 sides, with their grinding surfaces marked by the aforesaid 

 sculpture ; while the whole series is in close contact, 

 and forms one of the most efficient grinding machines 

 imaginable. 



To support the enormous weight of the carapace, which 

 in some of the larger kinds is considerably more than an 

 inch in thickness, special modifications are needed in the 

 internal skeleton. Here we find that nearly the whole of 

 the vertebrae are welded together, so that a large portion 

 of the backbone forms a continuous solid tube. The ver- 

 tebrae of the neck are also very short, and may be partially 

 united, so that the movements of the head must have been 



